A C63 AMG with is a pretty stout car once the M156 V8 is ECU tuned considering from the factory Mercedes detunes the car to protect the other 63 AMG M156 based vehicles in the lineup. A K04 turbocharger upgrade is not exactly a crazy turbo upgrade for the GTI yet the roll on race video below shows how potent a GTI with a K04 turbo and a stout tune can be. The GTI is running the GIAC 'extreme' K04 file and is able to pull this C63 AMG.

What tune is on the C63 AMG and what is it putting down to the wheels? Who knows, that is not specified. Regardless, this GTI wins this race and beats the C63 AMG decisively. The GIAC K04 Extreme tune definitely is working that turbo well.